This invention relates to chip separation techniques, and it relates, more particularly, to the separation of thin wafers such as the type generally using III-V compound semiconductor material.
Thin wafers, typically gallium-arsenide (GaAs) are presently being separated into dice by a number of conventional techniques. Presently, these approaches entail sawing, laser scribing, mechanical scribe and cleave, and chemical etch and cleave.
When thin GaAs wafers are sawed, chipping results along the sawed edges due to the brittleness of GaAs. Even under optimum conditions, smooth orthogonal edges are not produced which makes the handling of the chips more difficult. In addition, micro-cracking can occur during the sawing, which even if it doesn't lower the yield, will tend to decrease device reliability since such cracks are self propagating during stresses attendant through thermal cycling between the transitions of being off and on and vice versa. Although laser scribing of thin compound semiconductor material is able to produce better edges than sawing, it is well known that laser scribing can also generate micro-cracks as a result of the unavoidable localized heating of the crystalline material. This is perhaps best known when laser scribing is used on thin GaAs wafers.
There are numerous problems associated with the conventional separation technique known as scribing and cleaving particularly in thin GaAs wafer material. If the dice, for example, is oriented slightly off the major crystallographic axis, the cleavage lines tend to deviate from the desired separation pattern and may even occur through the active region of the devices lowering the yield. This problem becomes of increasing importance as the wafer diameter increases which is a pervasive trend in practically all semiconductor fabrication processing and materials for obvious reasons well known to those skilled in the device fabrication art. For example, GaAs field effect transistors (GaAs FETs) are usually fabricated with the major surfaces of the wafer corresponding to the (100) crystallographical plane wherein separation into dice having straight edges must be aligned perpendicular to the (110) planes.
The problem associated with the techniques of scribing and cleaving also occur during chemical etching and cleaving in addition to other problems. For example, chemical etching is known to produce undercutting of masked areas and rounding of the bottom of the separation pattern which increases the difficulty in obtaining proper cleaves.